In an optical communications system a wavelength division multiplexing (WDM) scheme can be employed. To this end, the optical communications system can comprise a demultiplexer, for example an AWG. The AWG comprises an input port for receiving a multiplexed optical signal comprising a number of wavelengths, each wavelength corresponding to a different communications channel. The AWG also comprises a plurality of output ports. The multiplexed optical signal is demultiplexed by the AWG, each communications channel being output by a different output port of the AWG.
Each of the different communications channels is separated from an adjacent communications channel by a predetermined channel spacing. The predetermined channel spacing is usually dictated by a technical standard relating to the WDM scheme employed. For example, the predetermined channel spacing presently specified by the International Telecommunication Union (ITU) is 100 GHz.
It is known that AWGs are susceptible to thermal variations that cause the optical energy of a first given channel at a first spatial position to encroach upon/drift into a second given channel at a second spatial position adjacent the first channel. The AWG is linear and so when shifting of the given channel to a second spatial position occurs within the AWG, such shifting occurs for all of the channels present.
As the predetermined channel spacing between adjacent channels decreases, the impact of wavelength drift between adjacent channels, i.e. cross-talk, increases.
In order to obviate cross-talk, fine temperature control of the AWG is necessary. Typically, a control loop is provided coupled to a thermoelectric cooler, the thermoelectric cooler being coupled to the AWG. In order to implement control, it is clearly necessary to monitor the temperature to provide a feedback signal. It is known to use a thermistor to provide such feedback of temperature of the AWG. However, using thermistors, it is difficult to control the temperature of the AWG to within less than one degree Celsius.